GT... by a margin... unless you take time to track down a GOOD Mongoose... they still make a couple models that are decent...
and dyno/gt are still better than most 'gooses.
my advice... find a decent Fit, WTP, Mosh,or any number of real BMX bikes... used! might be less than a walmart bike, and will be a much better ride... get the used bike... go through it, and wind up with a great bike for your son.
did you ask this question on the BMX forum? they will, i'm sure, have lots of better alternatives to offer you. late 80's, early 90's GT's are great bikes, too... and i've had some nice 'Gooses here, too...
heck... hit your local BMX shop up... see what they have, used.
and if your son isn't going for a "trick" bike, don't get a bike with stunt pegs... they are heavy, and just cause shin scars... as will the longer axles they screw onto.
look through your local Craigslist... you might be surprised at some of the great BMX deals you find...